(1) Field of the Invention
The present invention relates to apparatus for calcining gypsum and similar materials, and more particularly refers to an apparatus in which fuel used to generate heat for the calcining process is more efficiently utilized.
(2) Description of the Prior Art
Gypsum calcination may be carried out in kettles and may be operated either continuously or batchwise.
In the process for calcining gypsum, gypsum in the form of calcium sulfate dihydrate is heated and converted to calcium sulfate hemi-hydrate, more commonly known as stucco or plaster. A large amount of heat is consumed in the calcining process. A portion of the heat is used to decompose the dihydrate crystals and transform them to the hemi-hydrate crystals. Additionally, a substantial amount of heat is utilized to drive off the water released in the calcining process. Various means are utilized for providing the necessary heat. In U.S. Pat. No. 3,901,645, a heating system is disclosed involving the use of flat flame burners for heating the bottom wall of the kettle, and additionally a luminous wall furnace for heating the side wall of the kettle. However, when the heating means is applied solely to the walls of the kettle, the process is not as efficient as would be desirable.
In U.S. Pat. No. 3,871,829, a horizontal calcining apparatus is shown also utilizing a gas flame for heating the horizontally disposed kettle. There again only the surface of the kettle imparts heat to the gypsum content being processed.
In U.S. Pat. No. 3,378,246, a vertical kettle type of calcining apparatus is disclosed utilizing a heating chamber through which hot gasses are passed for heating the kettle. However, here too, only the walls of the kettle transmit heat to the gypsum content inside also resulting in a somewhat decreased efficiency in terms of heat utilized.
In U.S. Pat. No. 3,236,509, a calcining kettle is shown in two versions. In one version a conventional heating chamber is used which surrounds the exterior of the kettle and all the heat is transferred to the calcination mixture by conduction through the kettle walls and through flues passing through the kettle chamber. Since the rate of conduction through the kettle is somewhat limited, a considerable amount of heat is lost in the discharged stack gases. A second version is shown in which the combustion gases are directly introduced into the chamber of the kettle and directly discharged into the reaction mixture. However, here too a good deal of the heat is wasted when the stack gases are discharged into the atmosphere at an elevated temperature.
In U.S. application Ser. No. 807,264, filed 6/16/77, now U.S. Pat. No. 4,161,390, a calcining kettle is disclosed which utilizes both heating of the exterior of the kettle and simultaneously introducing combustion gases generated by a separate burner directly into the reaction chamber of the kettle. However, here too a considerable portion of the heat energy escapes with the stack gases.
In German, Auslegeshcrift No. 1,558,080, published Feb. 12, 1970, a calcining kettle is disclosed having a conventional heating chamber for heating the bottom and sides of the kettle. Additionally, fresh air is drawn in from the atmosphere and passed through a heat exchanger which is heated by the stack gas. As a result, some of the heat from the stack gas is recovered. However, much of the stack gas heat is wasted, since the fresh air must first be heated from ambient temperature to a temperature at which the contents in the kettle are maintained, and this heat is lost to the calcining process.